Compressive Strength Test of Concrete Cubes

Cube Testing of Concrete is a standard method used to determine the compressive strength of hardened concrete. It’s one of the most widely used tests in civil engineering for quality control on construction sites.

✅ Purpose of Cube Test

To ensure that the concrete mix used in construction meets the required compressive strength as per the design specifications (e.g., M20, M25, M30 grade, etc.).

📦 Cube Test Procedure

1. Sampling of Concrete

Fresh concrete is collected during casting.
Sample is taken as per IS 1199:1959.

2. Mould Size

Standard cube size: 150 mm × 150 mm × 150 mm
- For concrete with aggregate > 20 mm, standard cube is used.
- For aggregate ≤ 20 mm, 100 mm cube may be used.

3. Casting of Cubes

Fill the cube mould in 3 layers, each approximately 50 mm thick.
Tamp each layer 25 times using a standard tamping rod.
Finish the surface smoothly with a trowel.

4. Curing

Keep the moulded cubes in a moist environment for 24 hours.
After demoulding, cure the cubes in a water tank for 7, 14, and 28 days.

5. Testing (as per IS 516:1959)

After the curing period, the cube is tested in a compression testing machine (CTM).
Load is applied gradually at the rate of 140 kg/cm² per minute until failure.

📏 Calculation of Compressive Strength

$Compressive Strength (N/mm²)=Load at failure (N)Area of cube face (mm²)\text{Compressive Strength (N/mm²)} = \frac{\text{Load at failure (N)}}{\text{Area of cube face (mm²)}}$

For a 150 mm cube:
$Area=150×150=22500 mm2\text{Area} = 150 × 150 = 22500 \text{ mm}^2$

📅 Test Age

Common testing days: 7 days and 28 days
28-day strength is considered the characteristic strength.

📋 Acceptance Criteria (As per IS 456:2000)

Average strength of 3 cubes ≥ Target strength
No individual cube should be less than 85% of the grade strength

📘 Example

For M25 grade concrete:

Target strength = 25 N/mm²
3 cubes tested at 28 days:
- Cube 1: 26 N/mm²
- Cube 2: 25.5 N/mm²
- Cube 3: 24.2 N/mm²
- Average = 25.23 N/mm² → Acceptable (above target)
- Minimum = 24.2 → > 85% of 25 = 21.25, so still acceptable

🔧 Precautions

Proper mixing and compaction
Moulds should be oiled and properly aligned
Curing should be adequate
Test machine must be calibrated.

✅ Cube Test Checklist for Site Engineers / Technicians

S. No.	Activity	Check/Action	Remarks
1	Concrete Mix Grade	Confirm mix design (e.g., M20, M25, etc.)
2	Cube Moulds Condition	Cleaned, oiled, and free from deformation
3	Mould Size	150 mm × 150 mm × 150 mm
4	Sampling of Fresh Concrete	Taken as per IS 1199
5	Filling of Cube Mould	In 3 equal layers
6	Compaction	25 blows/layer using standard tamping rod
7	Surface Finishing	Trowel finish, flush with mould top
8	Curing Before Demoulding	Covered with moist hessian or plastic sheet	For 24 hours
9	Demoulding Time	After 24 ± 2 hours
10	Cube Identification Marking	Date, cube number, mix grade
11	Water Curing Duration	7, 14, 28 days
12	CTM Calibration Check	Ensure machine is calibrated
13	Test Age	Record curing age on test date
14	Testing Procedure	Load applied uniformly until failure	As per IS 516
15	Record Peak Load	Load in kN or N
16	Calculate Compressive Strength	Load ÷ Area of cube face
17	Evaluate Result	Check against design mix & IS 456

📄 Sample Cube Test Report Format

CONCRETE CUBE TEST REPORT
Project Name: XYZ Construction
Location: __________________
Client: ____________________
Tested By: ___________________
Lab/Agency: __________________
Report No.: __________________
Date: //____

S. No.	Cube ID	Date of Casting	Grade	Size (mm)	Test Age (Days)	Date of Testing	Load (kN)	Strength (N/mm²)	Result
1	C-01	10/06/2025	M25	150×150×150	7	17/06/2025	470	20.89	Pass
2	C-02	10/06/2025	M25	150×150×150	28	08/07/2025	600	26.67	Pass

Compressive Strength = Load (N) / Area (mm²)
Area = 150 × 150 = 22,500 mm²

Remarks: Cubes meet required strength criteria as per IS 456:2000.

Tested By: _____________
Approved By: _____________